@xDM

They wouldn't, but lets say there's a lag in action from a mod... Then all of you that know that it is a spoiler keep refferencing and identifying a single post as a spoiler then the chances of other members of the community reading it and becoming aware of it will increase exponentially. Otherwise, the post might have the chance of going unoticed to the great majority, and that's what we want. That's only one of several advantages of having that rule in place.